1. Field of the Invention
The invention relates to a technique of forming an image on a recording medium which is supported by a support member and which is to be fed to a photoreceptor, and more particularly to a technique of adjusting the position of the support member for a desired position of an uppermost recording medium on the support member.
2. Description of the Related Art
In general, image forming apparatuses such as printers are each configured to include a feeder tray for feeding a recoding medium in the form of a recording sheet of paper, for example. The feeder tray includes a sheet pressure plate on which a stack of recording sheets are received, or onto which a stack of recording sheets are loaded, and a pick-up roller for picking up from the feeder tray an uppermost one of the recording sheets stacked on the sheet pressure plate.
In the above-mentioned image forming apparatuses, the sheet pressure plate is supported to allow a pivotal movement thereof about the axis of one of both ends of the sheet pressure plate remote from the pick-up roller. One of the both ends of the sheet pressure plate adjacent to the pick-up roller is spring biased toward a feed roller. This arrangement permits the uppermost recording sheet to be pressed onto the pick-up roller and to be fed because of rotation of the pick-up roller.
The sheet pressure plate bears the weight of the recording sheets on the sheet pressure plate, the magnitude of which depends on the number and the sizes of the recording sheets on the sheet pressure plate.
For the above reasons, the feeder tray having the above-described configuration introduces variations in magnitude of a pressing force acting between the recording sheets received on the sheet pressure plate and the pick-up roller in pressing contact with each other, depending on the number and the sizes of the recording sheets received on the sheet pressure plate.
Such variations in pressing force may cause an unintended event such as double sheet feed (multi-sheet feed) or sheet misfeed (sheet miss feed) of the recording sheets from the feeder tray. More specifically, if the pressing force is larger than desired, double sheet feed occurs in which multi recording sheets are fed out together from the feeder tray in superposed relationship, while, if the pressing force is smaller than desired, sheet misfeed occurs in which no recording sheet is fed out from the feeder tray despite of rotation of the pick-up roller.
In the feeder tray having the above-described configuration, each time that the individual recording sheets are picked up from the feeder tray one sheet at a time by the pick-up roller, the uppermost one of the recording sheets on the sheet pressure plate is brought into pressing contact with the pick-up roller, with the sheet pressure plate being slightly oscillated. For this reason, the above-described configuration may also invite a noisy operation of the feeder tray during its continuous feeding operation of recording sheets.
For example, Japanese Patent No. 2698535 discloses a configuration in which a drive source moves a sheet receiver receiving sheets by virtue of a driving force applied from the drive source, from a position with the sheets on the sheet receiver being apart from a pick-up roller, to a position with an uppermost one of the sheets on the sheet receiver being in contact with the pick-up roller, to thereby retain the uppermost sheet at a given position relative to the pick-up roller.